Modern communications systems transmit and receive information by modulating a radio frequency (RF) carrier signal with an information signal. The information signal can be at a much lower frequency than the RF signal. Such systems can then demodulate the RF signal to recover the information signal.
RF receivers typically use heterodyning to convert a received RF signal to a lower frequency signal, known as the intermediate frequency (IF) signal, to make it easier to filter. Generally, heterodyning refers to a process of mixing (or multiplying) the RF signal with a local oscillator (LO) signal. The mixing process translates the RF signal to sum and difference frequencies. If the LO signal is provided with a frequency that mixes a selected one of the sum and difference frequencies of a desired channel to a fixed IF, then the mixed signal can then be subsequently filtered using a fixed-frequency IF filter that can be made high quality since the filter doesn't have to be tunable.
There are many known architectures for practical IF filters. However certain characteristics of these filters such as passband center frequency and bandwidth will differ from their ideal characteristics due to variations in component values. The problem becomes worse when the filter is implemented in a single integrated circuit, since on-chip components usually have wide tolerances of about 20% and thus vary over a wider range than comparable discrete components.
One solution to the problem is to perform calibration. During calibration, the values of circuit components are varied and the operation of the resulting IF filter is measured. The calibration operation continues until “optimum” values of the components are determined.
The calibration operation can cause problems of its own. The local oscillator can provide calibration tones that are input to the IF filter to measure its frequency response. The calibration tone can then be varied in frequency while the response of the filter measured, and the components of the IF filter can be adjusted until the filter is accurately tuned to the desired IF. Local oscillators typically use phase locked loops that multiply a relatively low frequency reference clock signal to a higher frequency suitable as the LO signal. If the LO is used to provide the calibration signal, then the PLL would have to re-lock every time the calibration tone frequency is changed, causing an unacceptably long calibration operation.